The present invention relates to an exhaust gas purifying system in which the amount of the secondary air supplied to the exhaust gas system of the internal combustion engine is regulated in accordance with the output signal of an air-fuel ratio detector so that the exhaust gas is purified by a three-way catalyst.
In the conventional systems of this type, in view of the fact that the three-way catalyst displays the optimum purifying ability at or about a predetermined (stoichiometric) air-fuel ratio, the air-fuel ratio in the exhaust system after mixture of the exhaust gas and the secondary air is detected by an air-fuel ratio detector and the amount of the secondary air to be supplied is controlled by feedback by an air control valve in such a manner that the air-fuel ratio in the exhaust system reaches a predetermined value.
The air pump making up a source of the secondary air is driven by the engine. Even though the displacement, i.e., the opening of the air control valve is controlled, the variation in the output of the air pump causes a change in the amount of the secondary air, thereby making it impossible to control the air-fuel ratio accurately. In order to obviate this problem, a system has been proposed in which a pressure regulator means is provided for adjusting the differential pressure between the upstream and the downstream of the air control valve at a predetermined value not related to the output of the air pump. Such a system still has a problem unsolved. Even though the opening of the air control valve is controlled by the output signal of the air-fuel ratio detector, the delay time for the secondary air to reach the air-fuel ratio detector and detection delay time of the air-fuel ratio detector itself are required before the air-fuel ratio in the exhaust system, i.e., the amount of secondary air changes upon actuation of the air-fuel control valve and the change is detected and an output signal is produced by the air-fuel ratio detector. As a result, the problem of the system controlling the air control valve constantly is that the range of variation in the air-fuel ratio is undesirably great.